Very recently, molecular diagnostics has increasingly gained in importance. It has found an entry into the clinical diagnosis of diseases (inter alia detection of infectious pathogens, detection of mutations of the genome, detection of diseased cells and identification of risk factors for predisposition to a disease).
In particular, through the determination of gene expression in tissues, nucleic acid analysis opens up very promising new possibilities in the study and diagnosis of disease.
Nucleic acids of interest to be detected include genomic DNA, expressed mRNA and other RNAs such as MicroRNAs (abbreviated miRNAs). MiRNAs are a new class of small RNAs with various biological functions (A. Keller et al., Nat Methods. 2011 8(10):841-3). They are short (average of 20-24 nucleotide) ribonucleic acid (RNA) molecules found in eukaryotic cells. Several hundred different species of microRNAs (i.e. several hundred different sequences) have been identified in mammals. They are important for post-transcriptional gene-regulation and bind to complementary sequences on target messenger RNA transcripts (mRNAs), which can lead to translational repression or target degradation and gene silencing. As such they can also be used as biologic markers for research, diagnosis and therapy purposes.
Alzheimer's disease (AD), also known in medical literature as Alzheimer disease, is the most common form of dementia. Alzheimer's disease is characterised by loss of neurons and synapses in the cerebral cortex and certain subcortical regions and leads to a gross degeneration in these regions. In AD protein misfolding and aggregation (formation of so-called “plaques”) in the brain is caused by accumulation of abnormally folded A-beta and tau proteins in the affected tissues.
Early symptoms are often mistaken to be age-related problems. In the early stages, the most common symptom is difficulty in remembering recent events. When AD is suspected, the diagnosis is usually confirmed with functional tests that evaluate behaviour and cognitive abilities, often followed by imaging analysis of the brain. Imaging methods used for this purpose include computed tomography (CT), magnetic resonance imaging (MRI), single photon emission computed tomography (SPECT), and positron emission tomography (PET). In a patients already having dementia, SPECT appears to be superior in differentiating Alzheimer's disease from other possible causes, compared with the usual attempts employing mental testing and medical history analysis. A new technique known as PiB PET has been developed for directly and clearly imaging beta-amyloid deposits in vivo using a tracer that binds selectively to the beta-amyloid deposits. Beta-amyloid deposits. Recently, a miRNA diagnostic test from serum has been proposed (Geekiyanage et al., Exp Neurol. 2012 June; 235(2):491-6.)
Symptoms can be similar to other neurological disorders. Diagnosis can be time consuming, expensive and difficult. In particular, the reliable and early diagnosis of Alzheimer based on non-invasive molecular biomarkers remains a challenge. Till today, early diagnosis of AD remains a great challenge. So far, findings of an autopsy or biopsy represent the most reliable diagnostics for this common disease
The attempt to report the presence of beta-amyloid not only in the brain, but also in other tissues, e.g. the skin, showed only limited relevance for diagnosing AD. (Malaplate-Armand C, Desbene C, Pillot T, Olivier J L. Diagnostic biologique de la maladie d'Alzheimer: avancées, limites et perspectives. Rev Neurol 2009; 165:511-520). Thus, in the recent past, different imaging as well as in vitro diagnostic markers have been proposed in order to improve the AD diagnosis. Most importantly, biomarkers that can detect AD in pre-clinical stages are in the focus, however, such markers can so far be only reliably detected in cerebrospinal fluid (CSF). One prominent example is the combination of beta-amyloid-1-42 and tau. In addition, molecular genetics analyses of single nucleotide polymorphisms (SNPs) in the DNA of patients have been proposed to provide a risk estimation of the presence of AD. In addition to variants in genes, several studies have described an association between AD and genetic variation of mitochondrial DNA (mtDNA). Here, no consistent evidence for the relation of mtDNA variants and AD could be reported Hudson G, Sims R, Harold D, et al.; GERAD1 Consortium. No consistent evidence for association between mtDNA variants and Alzheimer disease. Neurology 2012; 78:1038-1042. However, although the heritability of AD is comparably high (60-80%), epigenetic and persistent factors also may play an important role.
Therefore, there exists an unmet need for an efficient, simple, reliable diagnostic test for AD.